U.S. patent number 4,688,009 [Application Number 06/733,430] was granted by the patent office on 1987-08-18 for triple-pane waveguide window.
This patent grant is currently assigned to Varian Associates, Inc.. Invention is credited to Patrick E. Ferguson, Andrew L. Nordquist.
United States Patent |
4,688,009 |
Ferguson , et al. |
August 18, 1987 |
Triple-pane waveguide window
Abstract
A waveguide window contains a central transverse pane of a
material with high dielectric constant such as alumina ceramic. The
central pane is an integral number of half-wavelengths thick. On
each side of the central pane and immediately adjacent it is a side
pane of material with relatively low dielectric constant such as
fused quartz. The side panes are odd numbers of quarter-wavelengths
thick. The dielectric constants of the side panes are preferably
the square root of the dielectric constant of the central pane. The
improved wave impedance matching provides a low wave reflection
over a wide frequency band.
Inventors: |
Ferguson; Patrick E. (Mountain
View, CA), Nordquist; Andrew L. (Mountain View, CA) |
Assignee: |
Varian Associates, Inc. (Palo
Alto, CA)
|
Family
ID: |
24947559 |
Appl.
No.: |
06/733,430 |
Filed: |
May 13, 1985 |
Current U.S.
Class: |
333/252;
333/35 |
Current CPC
Class: |
H01P
1/08 (20130101) |
Current International
Class: |
H01P
1/08 (20060101); H01P 001/08 () |
Field of
Search: |
;333/33,35,252 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul
Attorney, Agent or Firm: Cole; Stanley Z.
Government Interests
The U.S. Government has rights in this invention pursuant to
Contract No. DASG60-79-C-0005 between the U.S. Army and Varian
Associates, Inc.
Claims
What is claimed is:
1. A waveguide window comprising:
a section of hollow waveguide adapted to transmit a wave with
transverse electric field;
a first pane of dielectric having a first dielectric constant,
extending across the open cross-section of said guide;
a second pane of dielectric having a second dielectric constant
extending substantially across said cross-section, said second pane
having a surface adjacent a first transverse surface of said first
pane;
a third pane of dielectric having a third dielectric constant
extending substantially across said cross-section, said third pane
having a surface adjacent the second transverse surface of said
first pane, said transverse surfaces are planes perpendicular to
the direction of propagation of said wave;
the dielectric constants of said second and third panes being
substantially equal to the square root of said first dielectric
constant.
2. A waveguide window comprising:
a section of hollow waveguide adapted to transmit a wave which
possesses a transverse electric field perpendicular to the
direction of propagation of said wave;
a first pane of dielectric possessing a first dielectric constant
and a first pair of parallel surfaces spaced apart substantially an
integral number of half-wavelengths of said wave disposed across an
open cross-section of said waveguide with said first pair of
parallel surfaces perpendicular to the direction of propagation of
said wave;
a second pane of dielectric possessing a second dielectric constant
substantially lower than said first dielectric constant and a
second pair of parallel surfaces spaced apart substantially an odd
number of quarter-wavelengths of said wave disposed substantially
across said open cross-section with one of said second pair of
parallel surfaces adjacent one of said first pair of parallel
surfaces; and
a third pane of dielectric possessing a third dielectric constant
substantially lower than said first dielectric constant and a third
pair of parallel surfaces spaced apart substantially an odd number
of quarter-wavelengths of said wave disposed substantially across
said open cross-section with one of said third pair of parallel
surfaces adjacent the other of said first pair of parallel
surfaces.
3. The window of claim 2 wherein said second and third dielectric
constants are substantially equal to the square root of said first
dielectric constant.
4. The window of claim 2 wherein said wave has circular electric
fields.
5. The waveguide of claim 2 wherein said first pane is largely
aluminum oxide.
6. The waveguide of claim 5 wherein said second and third panes are
fused silica.
7. The window of claim 2 wherein said first pane is hermetically
sealed across waveguide and said second and third panes are not
sealed to said first pane.
8. The window of claim 7 wherein said second and third panes are
not hermetically sealed to said waveguide.
Description
FIELD OF THE INVENTION
The invention pertains to windows of dielectric material which are
commonly used to isolate a portion of a waveguide filled with gas
from another portion which is evacuated or filled with a different
gas. Such windows are typically made of panes of ceramic such as
aluminum oxide or beryllium oxide ceramic. Windows have also been
made of glass, fused quartz, single-crystal sapphire and thin mica.
The ceramic type windows are generally sealed across the hollow
cross section of the waveguide by metallizing the edges of the
ceramic and brazing to the metallic waveguide. The mica windows,
which are generally obsolete, were sealed to the waveguide by a
thin fillet of melted glass. Glass windows are sealed by melting to
special metal parts of the waveguide structure which have
coefficients of thermal expansion matching that of the glass.
PRIOR ART
Placing a dielectric window across a uniform waveguide always
creates some reflection of the wave, because the dielectric has a
dielectric constant higher than the gas or vacuum in the rest of
the guide. This means the wave impedance in the window material is
lower. The abrupt change in impedance for a wave entering the
dielectric inherently causes partial reflection of the wave. In the
mica windows mentioned above and in some thin glass windows the
thickness of dielectric may be made sufficiently small compared to
a guide wavelength that the reflection may be neglected or
cancelled by well-known matching techniques, such as reactive posts
in the waveguide.
When dealing with extremely high frequencies and high powers, the
window thickness becomes comparable to a guide wavelength and the
reflection, which creates a standing wave in the guide outside the
window, becomes an important disadvantage.
The first art toward eliminating the reflections consisted in
making the window of a thickness equal to one-half of the
wavelength of the transmitted wave in the dielectric-filled
waveguide. In an infinite cross section the wavelength in a
dielectric medium is reduced from that in free space by the square
root of the dielectric constant. In a waveguide the reduction is
greater than this because the cut-off frequency of the waveguide is
also reduced. In the half-wavelength thick window the reflection
from the fornt surface is exactly cancelled by a reflection from
the rear surface when the wave leaves the dielectric. Thus for that
particular thickness and frequency there is no reflection. However,
as the frequency is changed from that for which the window is
one-half wavelength the amount of reflected energy increases
approximately linearly with the frequency deviation from that
central value. Therefore the frequency band over which the half
wave window has negligible reflection is limited to a value which
is often unsuitably small.
An improvement in band width is described in U.S. Pat. No.
3,345,535 issued Oct. 3, 1967 to Floyd O. Johnson and Louis T.
Zitelli. The invention described therein is to place a second half
wave window at a distance from the first window of one-fourth of a
guide wavelength in a guide filled with vacuum or gas. FIG. 1
illustrates this prior art. The hollow waveguide 10 may have a
number of cross sectional shapes, such as rectangular, circular,
ridged, or coaxial (not shown). The two dielectric panes 12 and 14
are exactly alike. At the center of the designed frequency band
they are each one-half of the wavelength in the dielectric filled
guide .lambda..sub.gd thick and are spaced by one-quarter of the
wavelength in the empty waveguide .lambda..sub.go.
The broad-banding can be calculated from simple waveguide theory.
Some help in understanding the effect is by analogy to resonant
circuits. The waves inside the panes are partly standing waves and
partly traveling waves. Due to the standing wave portion each
window has some analogy to a resonant circuit. Coupling the two
resonances in the right phase produces a broad-banding analogous to
coupled lumped-constant circuits. The pass band has a considerably
flatter extent than for a single half wave window.
Other prior art pertinent to the invention is the well-known
canceling of the reflection at a single discontinuity between the
media of different dielectric constants such as air and glass by a
layer one-quarter wavelength thick of a dielectric with dielectric
constant equal to the geometric average of the dielectric constants
of the two media. This system is widely used to reduce optical
reflections from glass surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic section through the axis of a prior art
waveguide window assembly as described above.
FIG. 2 is a schematic section through the axis of a waveguide
window assembly embodying the invention.
SUMMARY OF THE INVENTION
An object of the invention is to provide a waveguide window having
very high power-handling capability and very wide frequency
bandwidth.
A further object is to provide a window with protection against
waveguide arcs.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The essence of the invention is illustrated by FIG. 2. Across the
hollow interior of a waveguide 10' is a pane of dielectric material
16 having relatively high dielectric constant. Suitable materials
for extremely high powers and frequencies are aluminum oxide
ceramic, beryllium oxide ceramic, single-crystal sapphire and fused
quartz. Pane 16 is typically hermetically sealed across waveguide
10' by metallizing the dielectric via well-known processes such as
sintering a powdered molybdenum-manganese mixture to the edge
surfaces which are subsequently brazed to the waveguide. At the
center frequency, pane 16 has a thickness of one-half the
wavelength in the dielectric-filled waveguide .lambda..sub.gdl
where dl is its dielectric constant.
In contact with the exposed faces of pane 16 are a pair of panes 18
and 20 of materials having lower dielectric constants d2 and d3
than central pane 16. Panes 18 and 20 are preferably of a thickness
equal to one-fourth of the wavelength at the desired center
frequency in the waveguide filled with the material of the
respective panes. The dielectric constants d2 and d3 of panes 18
and 20 are chosen to match the waves in the input waveguide 22 and
output waveguide 24 to the wave in the central pane 16. At the
center frequency the wave in central pane 16 is then a pure
traveling wave, whereby the electric field in pane 16 is minimized.
Also, the window assembly has reduced reflections over a wider
bandwidth than prior-art windows. In this respect it is somewhat
analogous to a triple tuned circuit. An experimental window in
which the central pane was an alumina ceramic and the side panes
were fused quartz exhibited a voltage standing wave ratio (VSWR)
less than 1.5 over a ten percent bandwidth.
The dielectric constant of fused quartz, 3.8, is not exactly the
square root of that of high-alumina ceramic, about 9.0.
Nevertheless, it seems to be close enough to provide a well-matched
window.
An advantage of the inventive window construction using quartz side
panes is that it is not necessary to make a hermetic seal of the
quartz to the metallic waveguide. The outside panes 18, 20 may be
only mechanically constrained in place, by methods not shown. Since
quartz has an extremely low coefficient of thermal expansion and is
mechanically somewhat weak, it has proven to very difficult to make
a quartz-to-metal seal without intermediate grading glasses. Thus,
pure quartz windows have not been widely used.
Another advantage of the inventive window is in protection from
waveguide arcs. In a gas-filled waveguide carrying high
continuous-wave power, an rf voltage breakdown causes an arc across
the guide which travels toward the power source at a speed which
increases with the power level. If the arc reaches the output
window of the microwave generator tube, its intense localized heat
can melt or thermally crack the window, destroying the tube. In the
prior art, it was known to place a second window outside the
hermetic vacuum window to stop the arc's progress, at least
temporarily. The fused silica pane of the inventive window can
provide this added function. Fused quartz has very low thermal
expansion, so is highly resistant to cracking by heat shock. Since
the matching quartz pane may not be sealed to the central hermetic
pane, its failure alone will not cause failure of the tube.
The above described window is a preferred embodiment. Other
structures and materials may be used within the scope of the
invention. The central pane may be any whole number of
half-wavelengths thick. The outside panes may be any odd number of
quarter-wavelengths thick. Adding a half-wavelength to a pane
thickness causes the wave reflected on leaving the pane to arrive
at the entry surface in the same phase.
The scope of the invention is to be limited only by the following
claims and their legal equivalents.
* * * * *